Proper lighting is a critical aspect of RES keeping and is generally misunderstood and overlooked. Normally, natural sunlight provides beneficial elements necessary for healthy turtles. There are three main elements provided by correct lighting: UVA rays, UVB rays and heat. For your turtle, UVA exposure is essential for normal behavior including activity, feeding and mating. UVB rays are crucial for your turtle’s health and development. UVB is necessary for the production of the vitamin D3 that is required for the metabolization of dietary calcium. Heat entices your turtle to bask. This warmth elevates their metabolism along with their digestive and immune systems. Drying off also helps hardening of the shell, scute shedding as well as minimizing algae growth and waterborne infections.

Sliders are diurnal, meaning they are active during the day rather than at night. The length of daylight or “photoperiod” is a guide you should use to determine how long your habitat's lights should be on. Mimicking your seasonal photoperiod is beneficial and the use of a timer is a huge help for making this easier. Lighting should be on for around 12 hours a day, depending on the time of year. Too little or too much light will disrupt your turtle’s activity and sleep cycles.

Make sure all lights are firmly attached and using the correct wattage within the limits of their fixtures. Your RES should not be able to touch them and they should not become wet. To ensure that maximum UVB is absorbed, it will be necessary for fluorescent UVB bulbs to be about 10-12 inches away and adjacent to the basking heat source. They also need replacement every 6-10 months. The UVB rays should not have a barrier between them and your turtle. Glass, acrylic and plastic block UVB while screens with grids under 1/2" can obstruct a good portion of UVB rays.

UVA Rays

As mentioned before, UVA exposure promotes normal behavior including activity, feeding and mating. UVA rays have UV wavelengths of 380–315 nm and are also referred to as Long Wave or "blacklight". These wavelengths are shorter than that of visible light though turtles can see into this range. This is the most abundant type of UV and it can penetrate glass. For people, this type of UV causes the aging of skin and wrinkles.

Obviously, sunlight provides the optimal source of UVA and UVB. Incandescent bulbs provide limited to moderate UVA and heat but no UVB rays. They might come as ordinary household light bulbs, flood lights, spot lights and halogen lamps. Special compact fluorescents produce UVA and UVB light but do not generate enough heat for basking. Mercury vapor bulbs can produce large amounts of UVA, UVB and heat. Be wary of bulbs that are “Full-Spectrum” since they may not necessarily have the correct UVA, UVB or heat. Do not use a bulb that is described as a "blacklight" or BLB (blacklight bulb).

UVB Rays

UVB’s primary importance for your RES is stimulating the natural production of the vitamin D3. This vitamin is required for the metabolization of dietary calcium and together with the calcium, plays a roll in the prevention of illnesses such as Metabolic Bone Disease. UVB rays, also out of the visible range, have UV wavelengths of 315–280 nm which are also referred to as Medium Wave. UVB causes sunburn and skin cancer in people. Exercise caution when looking at these bulbs directly.

Direct and unfiltered sunlight is the best source of UVB. For indoor setups, turtle keepers need to rely on providing UVB through special bulbs. These bulbs are specially manufactured to emit UVB rays and have special glass casings that allow the rays to pass through. The most popular type of UVB bulbs is fluorescent, which come in different sizes, strengths, but produces very little heat since they are low-wattage. Also available are the more expensive mercury vapor bulbs. These produce large amounts of UVA, UVB and heat and are therefore not suitable for smaller aquariums.

Since UV is harmful to people, certain elements are added to glass and plastic to block UVB rays. This is why keeping a tank near a window is ineffective, while creating a dangerous overheating situation. Window screens also block some UVB rays and open windows exposes your turtles to cold breezes. Water also filters out UVB rays, which is why it is recommended that you place your UVB bulb directly over the basking area.

Note: UV radiation is known to cause certain types of cancer. Even though there is minimal risk, do not look directly into UVB lights at close distances and observe warning labels on packages.

Heat is a form of energy that can come from various sources. Technically speaking, electromagnetic radiation from visible light and infrared radiation are the most common sources of heat used for basking. Thermal radiation, for example, is created through the infrared radiation from an emitter or light from an incandescent bulb.

Providing basking warmth is probably the easiest aspect of turtle keeping. This warmth encourages basking in which a turtle would come out of the water to dry off and warm itself. Outside, it would do this under the Sun’s rays while absorbing UVA and UVB rays. For turtles kept indoors, using an incandescent bulb, a ceramic bulb or a mercury vapor bulb are several options to provide basking warmth. It is important to note that the basking area only needs to be 10 degrees F warmer than the water temperature to entice basking. Basking and other related issues are discussed thoroughly in their own sections.